Symbiotic nitrogen fixation takes place in particular plant root organs named nodules. Nodule formation on plants of the Leguminosae family is a result of consecutive interactions with bacteria of the Rhizobiaceae family (rhizobia). The interaction is considered mutually beneficial.
The bacteria within the nodule cells gain the ability to fix nitrogen gas by means of their nitrogenase enzyme complex and supply the host plant with the reduced nitrogen for plant growth. The plant provides photosynthates to the bacteria and a microaerobic niche for the oxygen-sensitive nitrogenase. Nodule development is induced by lipochitooligosaccharide signals of rhizobia, called Nod factors.
Nodules are formed on a particular host only in response to compatible rhizobia producing Nod factors with the adequate chemical structure (Dénarié, J., Debellé, F. & Promé, J.-C. (1996) Annu. Rev. Biochem. 65, 503-535). This is one of the major causes of the generally pronounced host specificity in Rhizobium-legume symbiosis. Nod factors induce cell divisions in the root cortex and successive divisions lead to the formation of the nodule primordium. Simultaneously, the rhizobia enter the host plant via the root hairs through the formation of tubular structures called infection threads which traverse the root epidermis and cortex and then the nodule primordium.
Rhizobia are released from infection threads in the cytoplasm of postmitotic nondividing cells by endocytosis. The term “bacteroid” refers to these intracellular membrane encapsulated bacteria. In legumes of the Papilionoideae subfamily, the nodules can be of either the determinate or the indeterminate type (Franssen, H. J., Vijn, I., Yang, W. C. & Bisseling, T. (1992) Plant Mol. Biol. 19, 89-107). In the case of determinate nodules, the initial cell division activity required for nodule primordium formation ceases rapidly and therefore the determinate nodules contain no meristem. Differentiation of infected cells occurs synchronously and the mature nodule contains symbiotic cells with a homogenous population of nitrogen-fixing bacteroids (Franssen, H. J., Vijn, I., Yang, W. C. & Bisseling, T. (1992) Plant Mol. Biol. 19, 89-107). Legumes such as bean (Phaseolus vulgaris) or Lotus japonicus form this type of nodules.
In contrast, cell division activity in the indeterminate nodules is maintained and forms an apical meristem (nodule zone I). Because the size of the meristem is constant, cell division activity and production of new sets of meristematic cells are balanced with the exit of the same number of cells from the mitotic cell cycle. These postmitotic cells are unable to divide and enter the nodule differentiation program. The infection thread penetrates into the submeristematic cells and liberates the rhizobia. In the infected cells, both partners differentiate progressively along the 12-15 cell layers of the infection zone (or zone II), ending in the formation of nitrogen fixing cells that will constitute the constantly growing nodule zone III (2). Legumes of the galegoid clade (such as Medicago spp., Vicia sativa, and Pisum sativum) are examples of plants forming indeterminate nodules.
In Medicago truncatula nodules several hundreds of peptide coding genes are expressed (Kevei et al., 2002, Mol. Plant-Microbe Interact., 15, 922-931; Mergaert et al., 2003, Plant Physiol. 132, 161-173; Alunni et al., 2007, Mol. Plant-Microbe Interact. 20, 1138-1148). Production of these peptides in the infected plant cells and their targeting to the bacteroids (Van de Velde et al., 2010) correlate with irreversible terminal differentiation of bacteroids involving genome amplification, cell elongation and branching, loss of cell division capacity and increase in membrane permeability (Mergaert et al., 2006, Proc. Natl. Acad. Sci. USA. 103(13):5230-5).
Natural products are the sources of most antibiotics, however the antimicrobial potential of plant peptides has been largely unexplored.
Nowadays, we face the danger of the emergence of antibiotic resistant bacteria as a consequence of misuse of traditional antibiotics that places selection pressure on the bacterium strains. Antimicrobial peptides are rarely used as antibiotics. Moreover, antibiotics such as polymyxins (cyclic peptide with hydrophobic tail) have adverse effects such as nephrotoxic and neurotoxic (Falagas M E, Kasiakou S K; February 2006; “Toxicity of polymyxins: a systematic review of the evidence from old and recent studies”. Crit Care 10 (1): R27) and, though effective, are therefore used only if less toxic antibiotics are ineffective or are contraindicated. Its typical use cases are infections with strains of Pseudomonas aeruginosa or Enterobacteriaceae species that are highly resistant to other types of antibiotics such as cephalosporins. Further, polymyxins are not absorbed from the gastrointestinal tract and therefore must be administered intravenously and active only against Gram−bacteria.